The Homestead Act
Normally USA should easily have 40 prestige by 1862 due to prestige from the Mexican-American wars, colonizing the west and the civil war.
The decision will give USA a +50% modifier to assimilation and immigrant attraction for the rest of the game. USA is already by far the nation, that receives the most immigrants due being in the new world, and this will only solidify the position as the number 1 destination.
All in all it is one of the best and most powerful decisions in the game.